Nested lists

Unit tests and fix to Parser.pm to preserve nestings in R lists (aka "generic vectors"). The nesting is done in Perl via references to embedded arrays.

Example:

    R: list(list(1, 2:4, list('a', 456, 'b'), 'foo')
    List of 1
     $ :List of 4
      ..$ : num 1
      ..$ : int [1:3] 2 3 4
      ..$ :List of 3
      .. ..$ : chr "a"
      .. ..$ : num 123
      .. ..$ : chr "b"
      ..$ : chr "foo"

    Perl: ( [1, [2, 3, 4], ['a', 456', 'b'], 'foo'] )
    0  ARRAY
       0  1
       1  ARRAY
          0  2
          1  3
          2  4
       2  ARRAY
          0  'a'
          1  456
          2  'b'
       3  'foo'